This invention relates generally to novel polymeric phosphorus-nitrogen compounds and a method of preparing the same and, specifically, to polymeric phosphorus-nitrogen compounds having alkyl/aryl side units bonded directly to the phosphorus atoms through carbon-phosphorus bonds.
Synthetic polymers have revolutionized modern man's lifestyle in many ways, contributing greatly to the technological advances which have taken place in recent years. There exists a need, however, for new synthetic polymers having properties not found in conventional polymers having organic backbones. For example, most synthetic polymers having organic backbones burn and evolve smoke, reducing their desirability as textile fibers or household objects. Although flame retardant additives have been developed, these additives are expensive to use and often have toxic properties. Many of the known synthetic organic elastomers harden and degrade in the atmosphere, especially at high temperatures, and are attacked by oils, hydrocarbon fuels, and industrial solvents.
Although various inorganic backbone polymers have been investigated in the hope of overcoming these and other problems, only the silicones have achieved commercial significance to date. Inorganic backbone polymers based on alternating phosphorus and nitrogen atoms, referred to as polyphosphazenes, were at first thought to be cross-linked and hydrolytically unstable. Thus, although the thermal conversion of hexachlorocyclotriphosphazene to poly(dichlorophosphazene), also known as "inorganic rubber", has been known for over 80 years, the resulting material degrades with prolonged exposure to moisture, limiting its practical applications.
Attempts were made to stabilize poly(dichlorophosphazene) by replacing the chloride atoms with organic substituents. These attempts were unsuccessful until, in 1965, H. R. Allcock and his colleagues at Pennsylvania State University prepared stable polyphosphazenes by the melt ring opening polymerization of hexachlorocyclotriphosphazene (trimer) followed by reaction of the soluble poly(dichlorophosphazene) which was formed with alkoxides such as sodium methoxide, ethoxide, or phenoxide, or with amines, such as aniline, piperidine or dimethylamine. The substituted products which resulted were hydrolytically stable elastomers or flexible thermoplastics of the general formula: ##STR1## where R is an alkyl group, Ar is an aryl group, and "n" ranges as high as about 15,000.
The polyphosphazenes produced were hydrolytically stable but have side units bonded to the phosphorus atoms through oxygen or nitrogen atoms and not by direct carbon-phosphorus bonds. These phenoxy, alkoxy, or aryloxy side units are ionizable or displaceable and destabilize the polymers thermodynamically, leading to depolymerization at temperatures above about 200.degree. C. Also, since the method involves first preparing a poly(dihalophosphazene) followed by nucleophilic displacement of the halogens along the chain, the substituent at phosphorus must be introduced after polymerization. It was not possible to incorporate the desired substituents before polymerization.
In 1977, Allcock and others succeeded in producing partially alkylated/arylated, mixed substituent polyphosphazenes of the general formula: ##STR2## where R is alkyl/aryl and "n" is in the range of about 10,000 to 15,000. The method involved reaction of Grignard and other organometallic reagents with high molecular weight fluorocyclophosphazenes such as high molecular weight poly(difluorophosphazene). However, attempts to achieve full replacement of the fluorine resulted in shortening of the polymer chains. The polymers produced by this method continued to be subject to attack through the trifluoroethoxy side unit. Allcock's partially alkylated/arylated polymers were also marked by a random substitution pattern along the polymer backbone.